Conventionally, a high-frequency heating device of the above type as represented by an electronic oven employs a high-frequency energy generating system which consists of a magnetron and a power source unit for driving the magnetron. A permanent magnet type magnetron and a ferroresonance type power transformer have been practically used for a long time. In recent years, with improvements in semiconductor technology, a switching type power source was proposed and put to practical use in place of the conventional ferroresonance type transformer achieve improved functional development, compact and light weight design, and cost reduction of the high-frequency heating device. In other words, use of a switching type power source enables a continuous proportional control of the microwave output and permits compact and light weight designing of the high-voltage transformer, which also leads to achieving high-performance heating with high-grade control, compact and light weight designing, and cost reduction of the high-frequency heating device.
FIG. 11 shows the circuit construction of a conventional high-frequency heating device. In FIG. 11, a commercial AC power source 1 is rectified by a rectifier 2 into a DC power and then converted into a high-frequency AC power by an inverter circuit 3 consisting of capacitors and a semiconductor switching element. The high-frequency AC power obtained by the inverter circuit 3 is applied to a transformer 4. The transformer 4 comprises a primary winding W1 to which the high-frequency AC power output produced in the inverter circuit 3 is applied, a secondary winding W2 for yielding a high voltage through a voltage step-up, and a tertiary winding W3 for yielding a low voltage.
The high AC voltage yielded in the secondary winding W2 is converted into a high DC voltage by a voltage doubler rectifier circuit 5 and is applied across the anode and the cathode of a magnetron 6 to thereby activate the magnetron.
The low voltage yielded in the tertiary winding is applied to the cathode of the magnetron 6 to heat the filament.
The magnetron 6 exhibits a non-linear characteristic as shown in FIG. 12. In a condition where the filament of the magnetron 6 is sufficiently heated, the magnetron starts to oscillate when the voltage VAK applied across the anode and the cathode thereof reaches about -4 kV, with which the voltage VAK across the anode and the cathode is clipped at -4 kV to reduce the impedance of the magnetron 6 to about several kiloohms. On the contrary, in a condition where the filament is not heated, or the voltage VAK across the anode and the cathode is lower than about -4 kV, the magnetron 6 is set in a non-oscillating condition and the impedance thereof remains virtually infinite. FIGS. 13 (a), (b), and (c) show the mutual relationship between the voltage VAK applied across the anode and the cathode, a current I.sub.f flowing through the filament, and the temperature of the filament in a period from the activation time of the inverter 3 to the oscillation time of the magnetron 6. At the activation time, a greater filament current I.sub.f1 flows in order to rapidly increase the filament temperature T.sub.f to a rating temperature T.sub.f2. The tertiary winding W3 of the transformer 4 for supplying a current to the filament and the secondary winding W2 for yielding a high voltage to be applied across the anode and the cathode are provided in the same transformer. Therefore, the secondary winding W2 is compelled to yield such a high voltage as to form a voltage VAKS, which is higher than the oscillation voltage of generally -4 KV of the magnetron 6, across the anode and the cathode.
That is, it has been conventionally inevitable that a voltage which is much higher than the oscillation voltage of the magnetron 6 is applied across the anode and the cathode of the magnetron 6 at the time of activating the high-frequency heating device in order to rapidly increase the filament temperature T.sub.f to the rating temperature T.sub.f2. For example, the voltage VAKS across the anode and the cathode at the activation time is compelled to be -8 kV to -10 kV despite the fact that the oscillation voltage of the magnetron 6 is usually about -4 kV.
Therefore, it has been required to design the endurance voltages of the diode and capacitors used in the voltage doubler rectifier circuit 5, rectifying the voltage yielded in the secondary winding W2 of the transformer, so as to tolerate the high voltage generated at the activation time. The above also results in such problems that the dimensions of the capacitors and the diode must be increased and the reliability is significantly degraded due to heat concentration accompanied by the increase in the amount of diode elements for the purpose of increasing the endurance voltage of the diode having a stack construction.
The interval from the time the filament temperature T.sub.f permits the start of thermionic emission to the time the filament temperature reaches the rating temperature at which a sufficient thermionic emission is achieved (the interval from T.sub.f1 to T.sub.f2 in FIG. 13) corresponds to the time period where the magnetron 6 enters from the non-oscillation condition into the oscillation condition. In the above time interval, the filament temperature T.sub.f has not yet reached the temperature sufficient for thermionic emission from the filament necessary for normal oscillation of the magnetron 6, therefore, an abnormal oscillation referred to as "moding" occurs to result in the problem of shortening the life time of the magnetron 6.
On the contrary, when an activation starts with the output of the inverter circuit 3 reduced in order to suppress the voltage VAKS across the anode and the cathode at the activation time, the filament current I.sub.f is reduced to result in the drawback of significantly prolonging the time t.sub.s necessary for activating the magnetron.
As is well known, the output of the inverter circuit 3 can be easily adjusted by controlling the conducting time of a transistor 7. However, when the conducting time of the transistor 7 is controlled to adjust the power produced in the primary winding W1, the power yielded in the tertiary winding W3 also changes while obtaining a capability of controlling the output of the secondary winding W2 supplying a high voltage power, which also changes the filament temperature T.sub.f of the magnetron 6 to promote the deterioration of the filament to consequently result in the drawback of significantly reducing the reliability of the magnetron 6.
As a method of stabilizing the filament temperature T.sub.f with respect to adjusting the output of the magnetron 6, disclosed is a method of providing the tertiary winding W3 with an ferroresonance function in the Japanese Patent Publication No. 56-3636 and Japanese Patent Laid-Open Publication No. 03-057193. Although the above-mentioned conventional examples propose a solution for stabilizing the filament temperature T.sub.f of the magnetron 6, they are not always capable of securely stabilizing the filament temperature T.sub.f in a variety of conditions, thus providing no solution to the above-mentioned problems occurring at the time of activating the high-frequency heating device.